Process for heating solid materials, particularly for the calcination of limestone



Patented Aug. 31, 1954 PROCESS FOR HEATING SOLID MATERIALS, PARTICULARLY FOR THE CALCINATION OF LIMESTONE Hans Erich Werner Heiligenstaedt, Volklingen,

Saar Territory, assignor to Administration- Sequestre des Rochlingsche Eisenu. Stablwerk G. in. b. H., Vo1klingen,'Saar, a body corporate Application August 16, 1950. Serial No. 179,701

Claims priority, application Germany September 3, 1949 4 Claims.

The shaft furnace is a heat exchanger of great efiiciency, because the heating gas licks nearly all of the surface of the material to be heated. The quantity of heat which can be transmitted to the material, all conditions being unchanged, is increased proportionally to the decrease of the size of the grains, and becomes about ten times higher if the size of the fragments is reduced from 200 mm. to 25 mm. The time needed for the heating falls down even faster than the quantity of transmitted calories increases, because the heating of fragments of small size may be carried out with small differences of temperature between the surface of the outer part and the central part of the fragments. Therefore, the quantity of transmitted calories becomes twenty times higher when the size of the fragments is reduced from 200 mm. to 25 mm. Crushed lime stone of 25 mm, for instance, are calcinated twenty times faster than a fragment of 200' mm. The eificiency of a shaft furnace could therefore become twenty times higher, but, in such a case, the quantity of heat supplied must also become twenty times larger and this could not be realized with the conventional shaft furnace in which the solid materials are drawn out of the oven, as the increase of the heating calls for a corresponding increase of the speed of the gases passing through the shaft. The resisting effect of pile of products loaded in the shaft furnace, when compared with an empty furnace, brings down the speed of the gases passing through the furnace to 0.05 to 0.10 m. per second in case of a natural draught and to about 0.20 m. per second, in case of a normal forced draught.

With such speeds, only a small supply of heat can be obtained, about 20,000 calories per hour and per cubic meter of the capacity of the furnace in the conventional lime shaft-furnaces, which corresponds to a production of to kg. of lime per hour, and per cubic meter of the capacity of the furnace. If a twenty time larger production was contemplated with heating under favorable circumstances, the speed of the burned gases should be increased up to about four m. per second, thus bringing the pressure losses to i a value four hundred times higher, amounting to about 0.4 atmosphere for each meter of height.

Such pressures cannot be practically realized for the type of shaft-furnaces in which solid matefurnace out of operation. This temperature regulation could become economical only if the principle of circulation of the burned gases would be adopted, this is to say in the case of an additional increase of the quantity of gases passing through the furnace.

For these reasons, there has been no extension in the use of shaft furnaces in which the solid materials are drawn out of the oven, as their efficiency is contradictory with the good preliminary conditions of heat exchange, as above indicated.

As the quantity of gas corresponding to the efficiency increase to be obtained, can only be forced through the materials piled in the furnace if the circuit of these gases is shortened, it is suggested, according to the invention, to carry out in the furnaces having a shaft or a chamber in which the material to be calcinated moves by gravity, a transversal heating of these materials. With this suggestion, the required increase of efficiency is reached, as the thickness of the layer of the material can be reduced to a few decimeters therefore gaining the advantage of the above mentioned requirements for 'heat exchange. In such a case, the delivery of gas may be increased in the ratio between the height of the shaft-furnace and the thickness of the layer of the material piled in the furnace, as the gas has no more to travel through the entire height of the furnace (10 to 20 m.), but only through a layer of material the Width of which is only a part of said height.

The circulation of the burned gases, which is necessary for the carrying out of the invention, would highly increase the consumption of calories in the operation if said burned gases were, as usually, taken at the final stage of heat exchange. It is therefore suggested, according to the invention, to make use, for the circulation, only of the burned-gases coming from the zone of temperature essentially determinated in accordance with the heat consumption required to carry out the operation, as for instance, in lime ovens, the burned gases issued from the baking zone, at about 900. It is well known that it has already been suggested for the circulation, to take a portion of the burned gases under the throat of the conventional lime furnaces, but the heat of these circulating gases, 500 to600 C. indicates that they are coming from the zone of preheating.

As an example, a ring-shaped shaft-furnace according to the invention is described hereafter and shown in the annexed drawings.

Fig. 1 is a vertical section of the shaft furnace and Fig. 2 is a cross section ,of Fig. 1 along line AA.

The furnace is provided with a ring shaped shaft in which the piled upmaterialsrcomedown in succession through the zone of precalcination, the zone of highest consumption of heat, (zone of calcination) and through thecoolingzone.

The heating gas is introduced in the furnace through the injection 2, and it sucks up, by means of the discharge passage 3 of said injector, the combustion air which arrives in 4 and which, after being preheated in the cooling zone, passes intoa-lower chamber5orair mainlin whichare disposed theinjectors 5. Thecombustionis completed in the combustion chamber ,6. The burned heating, gas enters the shaft I through. a number of perforations 1, passes through the material in the shaft and leaves the shaft I, through perforations 8; it enters the middle chamber ..9 .from where .it .is conducted through the flue Hi, and the vertical draughtpipe H into the lower chamber 5.

The circulation of the largeamount of burned gases issued from the combustion zone and mixed, in the lower chamber:5,'with the air passing from 4 in said chamber 5,.is alsoinducediby thezinjectorz .dischargingrinto the passage} the heating gas at asuflicient pressure :to produce at the ;lower opening of said passage "a suction. When the furnaceiis heated with solid fuel, the combustion air is partly utilized as means for the suction through the injector. .ingportion of burned-gases, not used for thecir- ,culation, is used for heating the preheating zone. This portion passes again in :the shaft through ;holes;l 2,,crosses in a perpendicular way the piled up materials; and ;leaves the furnacethroughthe holes t3, the flue chamber l4 1 and the, pipe {5.

The -.-transverse passage through'the preheat- ;ingzonemay be omitted, and; theburned, gas, may

cross in counter-flow this zone if vsaidzone gis sufficiently low in the direction of movement of the materialto'be heated, to allow;passage of the heatingtgas through the materials in'said preheating zone-in spite ofthepressure existing in shaft.

-What;I claim is 1.;The process of calcining crushed limestone ,which comprises, establishing :a "vertical, .downrwardly-descending column of said limestone,

.said top and saidbottom of said column,.passing said combustiongases transversely through :said column and transversely withdrawing the coin- .bustion gases from (said column .at .a .point substantially horizontally opposite :the point of introduction of said ,gases whereby .said

combustion .gases .create in .said column 'ad- The .remainjacent said point of introduction a temperature zone of maximum heat consumption and said combustion gases are transversely withdrawn solely from said zone, passing a portion of the combustion gases from said zone of maximum heat consumption to a point in said column above said zone to create a pre-heating zone wherein the freshlimestone being charged to said column is pre-heated before it reaches said zone of maximum heat consumption, withdrawing said gases from said pre-heating zone, ,andre-circulating to said source of fresh combusticn gases onlycombustion gases withdrawn transversely from said zone of maximum heat consumption, whereby said transversely withdrawn combustion gases are mixed with fresh combustion gases and the resulting mixture is introduced into said column for transverse passage therethrough.

a2. Ashaft furnace for the heat treatment of crushedlimestone and like solid materials comprising, in combination, spaced-apart vertical walls defining a vertical channel for the continuous downward movement of the material being treated, said channel having an upper pre-heat- ,ing portion, an intermediate heating portion, and a lower cooling portion, second wall means spaced apart with respect to one of said firstvnamed walls, horizontal partition ,means connected tosaidsecond wall means and to said one of said first-named wallsto define a combustion chamber opposite said intermediate heating portion, thewallseparating said combustion chamber from said vertical chamber being provided with a plurality of apertures establishing com- ,munication between the intermediate heating ,portion ofsaidvertical channel and-the combustion chamber, partition means including the other of saidfirst-named walls defining a gasre- .ceiving chamber on {the side of said vertical channel opposite said combustion chamber,-and said wall separating ,said vertical-channel from said gasreceiving chamber being provided with apertures establishing communication between the intermediate heating portion of said channel and saidgas receiving chamber, whereby the apertures in said two wallsare substantially op- ;posite one another, meansdefining a combustion gas passageway .having a discharge side com- ,mum'cating with saidcombustion zone and an opposite suction side, nozzlesfor supplying com- .bustion gasdisposed with their outlets in said passageway facing saiddischarge side, whereby a suction eifectis developed onthesuction side of said passageway upon introduction of combustion .gas through said nozzles, air supply .means communicating with said cooling zone for .ldrawing incombustion air through said cooling zone, and conduit means providing. communicationbetweenthe suction side of said passageway .and said gas receiving chamber, whereby combustion gases from said vertical channel are drawninto said ,gasreceiving chamber and are thendrawn into said passageway for admixture with combustion-gases introduced through said nozzles for recirculation to said vertical channel containing the. material being treated.

3. A shaft furnace for the heat treatment of :crushed limestone and like solid materials comprisin :in combination, spaced-apart vertical walls defining a vertical channel ;-for :the con- *J IIHOI- S downwardmovement of the material be- .ing treatedhsaidipchanml having anupper pre- .heating. portion, an. intermediate heatingportion, andiailowercooling portion, second .wall means spaced apart with respect to one of said firstnamed walls, horizontal partition means connected to said second wall means and to said one of said first-named walls to define a flue gas collecting chamber opposite said upper preheating portion and a combustion chamber opposite said intermediate heating portion, the wall separating said combustion chamber from said Vertical chamber being provided with a plurality of apertures establishing communication between the intermediate heating portion of said vertical channel and the combustion chamber, partition means including the other of said firstnamed walls defining a gas receiving chamber on the side of said vertical channel opposite said combustion chamber, and said wall separating said vertical channel from said gas receiving chamber being provided with apertures establishing communication between the intermediate heating portion of said channel and said gas receiving chamber, whereby the apertures in said two Walls are substantially opposite one another, means defining a combustion gas passageway having a discharge side communicating with said combustion zone and an opposite suction side, nozzles for supplying combustion gas disposed with their outlets in said passageway facing said discharge side, whereby a suction effect is developed on the suction side of said passageway upon introduction of combustion gas through said nozzles, and conduit means providing communication between the suction side of said passageway and said gas receiving chamber, whereby combustion gases from said vertical channel are drawn into said gas receiving chamber and are then drawn into said passageway for admixture with combustion gases introduced through said nozzles for recirculation to said vertical channel containing the material being treated.

4. A shaft furnace for the heat treatment of crushed limestone and like solid materials comprising, in combination, spaced-apart concentric vertical circular walls defining a vertical annular channel for the continuous downward movement of the material being treated, said channel having an upper pre-heating portion, an intermediate heating portion, and a lower cooling portion, second wall means spaced apart with respect to one of said first-named walls, horizontal partition means connected to said second wall means and to said one of said first-named walls to define an annular flue gas collecting chamber opposite said upper pre-heating portion and an annular combustion chamber opposite said intermediate heating portion, the wall separating said combustion chamber from said vertical chamber being provided with a plurality of apertures establishing communication between the intermediate heating portion of said vertical channel and the combustion chamber, partition means including the other of said first-named walls defining a gas receiving chamber on the side of said vertical channel opposite said combustion chamber, and said wall separating said vertical channel from said gas receiving chamber being provided with apertures establishing communication between the intermediate heating portion of said channel and said gas receiving chamber, whereby the apertures in said two walls are substantially opposite one another, means defining a combustion gas passageway having a discharge side communicating with said combustion zone and an opposite suction side, nozzles for supplying combustion gas disposed with their outlets in said passageway facing said discharge side, whereby a suction effect is developed on the suction side of said passageway upon introduction of combustion gas through said nozzles, and conduit means pro viding communication between the suction side of said passageway and said gas receiving chamber, whereby combustion gases from said vertical channel are drawn into said gas receiving chamber and are then drawn into said passageway for admixture with combustion gases introduced through said nozzles for recirculation to said vertical channel containing the material being treated.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,669,012 Nordstrom May 8, 1928 1,895,284 Hay Jan. 24, 1933 2,037,809 MacMullin Apr. 21, 1936 2,113,522 Walker Apr. 5, 1938 2,451,024 Ellerbeck Oct. 12, 1948 2,470,543 Azbe May 17, 1949 2,523,835 Lepersonne Sept. 16, 1950 2,552,063 Robinson May 8, 1951 2,595,574 Grossman May 6, 1952 

